(1) Field of the Invention
The present invention relates to an optical pickup used for a semiconductor laser apparatus or an optical disc apparatus, and relates to a diffraction grating element that is a component of the optical pickup.
(2) Description of the Related Art
There are various types of optical discs meeting the respective standards, such as CD-ROM (Compact Disc-Read Only Memory), CD-R(Compact Disc-Recordable), CD-RW (Compact Disc-ReWritable), MD (MiniDisc), DVD-ROM (Digital Versatile Disc-Read Only Memory), and DVD-R (Digital Versatile Disc-Recordable). Optical disc systems each adopt a servo signal detection method that meets the standard of the type of optical disc used in the optical disc systems.
Such servo signal detection methods include the spot-size detection method (hereinafter referred to as SSD method) for detecting a focus error signal. The servo signal detection methods also include methods for detecting a tracking error signal, such as the 3-beam method, the push-pull method (hereinafter referred to as PP method), the difference push-pull method (hereinafter referred to as DPP method), and the differential phase detection method (hereinafter referred to as DPD method).
In recent years, the mainstream of the optical disc system is a complex system that can deal with a plurality of types of optical discs, such as a CD complex system that can deal with CD-ROM, CD-R, and CD-RW. Such complex systems are required to adopt both the DPP method and DPD method so that the tracking error signal can be detected from any of the different types of optical discs that have different pit depths.
Japanese Laid-Open Patent Application No. 11-296873 discloses an optical disc system adopting the DPD method. FIG. 1 of the present application is a perspective view showing an error detection part of a conventional optical disc system. FIG. 2 is a plane view showing a diffraction grating element of the conventional optical disc system. FIG. 3 is a plane view showing a light-sensitive element substrate of the conventional optical disc system.
As shown in FIG. 1, a laser beam emitted from a semiconductor laser (not illustrated) travels along an optical axis 101, reaches and is reflected by an optical disc 102. The optical disc 102, a diffraction grating element 103, and a light-sensitive element substrate 104 are arranged in the stated order along the optical axis 101. A reflected beam 105, which is a laser beam reflected by the optical disc 102, passes through the diffraction grating element 103 along the optical axis 101, and reaches the light-sensitive element substrate 104.
The diffraction grating element 103 includes, at the center thereof, a diffraction grating area 106 having a diffraction function and a lens function. As shown in FIG. 2, the diffraction grating area 106 is divided into two: a first area 106a; and a second area 106b, by a straight line 106c that intersects with the optical axis 101 and is parallel to a direction 107 that is tangent to a curve of a pit sequence of the optical disc 102 (hereinafter, the direction 107 is referred to as a tangential direction 107).
The first area 106a and second area 106b have the same diffraction angle and different diffraction directions. Accordingly, the reflected beam 105 that enters the first area 106a is divided into a plus-primary diffracted beam 108a+ and a minus-primary diffracted beam 108a−, and the reflected beam 105 that enters the second area 106b is divided into a plus-primary diffracted beam 108b+ and a minus-primary diffracted beam 108b−.
The first area 106a and second area 106b also have a lens effect with which the diffracted beams 108a− and 108b+ converge, and the diffracted beams 108a+ and 108b− diverge.
As shown in FIG. 3, four light-sensitive areas 109a+, 109a−, 109b+, and 109b− are formed on the light-sensitive surface of the light-sensitive element substrate 104. The light-sensitive areas 109a+, 109a−, 109b+, and 109b− are arranged so that a first straight line 110a, which runs through the center of the light-sensitive areas 109a+ and 109a−, intersects, approximately at the optical axis 101, with a second straight line 110b that runs through the center of the light-sensitive areas 109b+ and 109b−.
Each of the light-sensitive areas 109a+ and 109a− is divided into areas Ea1, Ea2, Eb, and Ec by three straight lines that are parallel to the first straight line 110a, where Ea1 and Ea2 are inner areas and Eb and Ec are outer areas. Similarly, each of the light-sensitive areas 109b+ and 109b− is divided into areas Ea1, Ea2, Eb, and Ec by three straight lines that are parallel to the second straight line 110b, where Ea1 and Ea2 are inner areas and Eb and Ec are outer areas.
As shown in FIG. 2, the plus-primary diffracted beam 108a+ and minus-primary diffracted beam 108a− divided in the first area 106a are diffracted in directions indicated as 111a+ and 111a− along the first straight line 110a, respectively. Also, the plus-primary diffracted beam 108b+ and minus-primary diffracted beam 108b31  divided in the first area 106b are diffracted in directions indicated as 111b+ and 111b− along the second straight line 110b, respectively. The diffracted beams 108a+, 108a−, 108b+, and 108b− are then enter the light-sensitive areas 109a+, 109a−, 109b+, and 109b−, respectively.
FIG. 4 shows how the focus error signal is detected by the SSD method. As shown in FIG. 4, a photoelectric conversion signal R+i is obtained from the inner areas Ea1 and Ea2 of the light-sensitive areas 109a+, a photoelectric conversion signal R+o is obtained from the outer areas Eb and Ec of the light-sensitive areas 109a+, a photoelectric conversion signal R−i is obtained from the inner areas Ea1 and Ea2 of the light-sensitive areas 109a−, and a photoelectric conversion signal R−o is obtained from the outer areas Eb and Ec of the light-sensitive areas 109a−. Also, a photoelectric conversion signal L+i is obtained from the inner areas Ea1 and Ea2 of the light-sensitive areas 109b+, a photoelectric conversion signal L+o is obtained from the outer areas Eb and Ec of the light-sensitive areas 109b+, a photoelectric conversion signal L−i is obtained from the inner areas Ea1 and Ea2 of the light-sensitive areas 109b−, and a photoelectric conversion signal L−o is obtained from the outer areas Eb and Ec of the light-sensitive areas 109b−.
In the above-mentioned case, the focus error signal FE is calculated using the following equation.FE=[(L+i+L−o)+(R+i+R−o)]−[(L−i+L+o)+(R−i+R+o)]  Equation 1
FIG. 5 shows how the tracking error signal is detected by the DPD method. As shown in FIG. 5, obtained are: a sum Ru+ of the photoelectric conversion signals of the inner area Ea1 and the outer area Eb of the light-sensitive areas 109a+, a sum Rd+ of the photoelectric conversion signals of the inner area Ea2 and the outer area Ec of the light-sensitive areas 109a+, a sum Ru− of the photoelectric conversion signals of the inner area Ea2 and the outer area Ec of the light-sensitive areas 109a−, and a sum Rd− of the photoelectric conversion signals of the inner area Ea1 and the outer area Eb of the light-sensitive areas 109a−. Also obtained are: a sum Lu+ of the photoelectric conversion signals of the inner area Ea1 and the outer area Eb of the light-sensitive areas 109b+, a sum Ld+ of the photoelectric conversion signals of the inner area Ea2 and the outer area Ec of the light-sensitive areas 109b+, a sum Lu− of the photoelectric conversion signals of the inner area Ea2 and the outer area Ec of the light-sensitive areas 109b−, and a sum Ld− of the photoelectric conversion signals of the inner area Ea1 and the outer area Eb of the light-sensitive areas 109b−.
In this case, the tracking error signal is detected by comparing the phases of “(Ru++Ru−)+(Ld++Ld−)” and “(Lu++Lu−)+(Rd++Rd−)”.
As explained above, the focus error signal is detected by the SSD method and the tracking error signal is detected by the DPD method, both from the plus/minus-primary diffracted beams 108a+, 108a−, 108b+, and 108b− diffracted in the diffraction grating area 106.
However, the above-described conventional optical disc system has the following problems.
As explained above, when the focus error signal is detected by the SSD method, signals from the inner areas Ea1 and Ea2 are always added up. As a result, it will be sufficient for each of the light-sensitive areas 109a+, 109a−, 109b+, and 109b  to be divided into three areas. On the other hand, when the tracking error signal is detected by the DPD method, signals from the inner area Ea1 and outer area Eb are always added up and signals from the inner area Ea2 and outer area Ec are always added up. As a result, it will be sufficient for each of the light-sensitive areas 109a+, 109a−, 109b+, and 109b− to be divided into two areas.
When both the SSD method and DPD method are adopted, however, each of the light-sensitive areas 109a+, 109a−, 109b+, and 109b− needs to be divided into at least four areas. That is to say, to adopt the DPD method, each of the light-sensitive areas 109a+, 109a−, 109b+, and 109b− needs to be divided into four, while it was sufficient for conventional systems to divide each area into three.
When, as is the case with the conventional optical system, the number of the divisional areas increases, various problems occur. For example, an existing light-sensitive circuit needs to be modified significantly, or the light-sensitive circuit becomes complex, or more external output terminals are required in the light-sensitive element substrate 104. The significant modification of the light-sensitive circuit removes from it the compatibility with other existing equipment. The complex light-sensitive circuit or the increase in the number of external output terminals causes the light-sensitive element substrate 104 and the package to grow in size. This prevents the optical disc system from being reduced in size and cost and simplified.
The following explains examples of connections, in the case where each of the light-sensitive areas 109a+, 109a−, 109b+, and 109b− is divided into four small divisional areas, between the divisional areas and the external output terminals. FIGS. 6–8 show examples of connections between the divisional areas and the external output terminals.
FIG. 6 shows a case where no arithmetic circuit is provided on the light-sensitive element substrate 104. In this case, as many external output terminals as there are divisional areas (that is to say, 16) are required.
FIG. 7 shows a case where arithmetic circuits are provided on the light-sensitive element substrate 104 to perform operations of four signals (L+i+L−o), (R+i+R−o), (L−i+L+o), and (R−i+R+o) relating to the focus error signal and four signals (Ru++Ru−), (Ld++Ld−), (Lu++Lu−), and (Rd++Rd−) relating to the tracking error signal. In this case, eight external output terminals are required in total.
FIG. 8 shows a case where the minimum number of external output terminals are required. In regards with the focus error signal, the arithmetic circuits perform operations of two signals [(L+i+L−o)+(R+i+R−o)] and [(L−i+L+o)+(R−i+R+o)] that are obtained by performing operations of the four signals (L+i+L−o), (R+i+R−o), (L−i+L+o), and (R−i+R+o).
Also, in regards with the tracking error signal, the arithmetic circuits perform operations of only two signals (Ru++Ru−) and (Lu++Lu−), and do not perform operations of the remaining two signals (Ld++Ld−) and (Rd++Rd−). That is to say, the tracking error signal is detected by comparing the phases of signal (Ru++Ru−) and signal (Lu++Lu−), by the DPD method. In this case, only four external output terminals are required in total.
It is noted that the arithmetic circuits of FIG. 8 are simpler than those of FIG. 7 and the number of external output terminals of FIG. 8 is half that of FIG. 7. In regards with detection of the focus error signal, however, the allowable range for the amount of signal per terminal in FIG. 8 should be two times that of the original one. Otherwise, the terminal output is saturated. Also, in regards with detection of the tracking error signal, the amount of signal that can be used to detect the tracking error signal in FIG. 8 is half the original one.
In addition, if the arithmetic circuits of FIG. 8 is applied to a conventional complex system, the PP method should be additionally adopted. This may make the arithmetic circuits more complex or increase the number of external output terminals. It should be noted here that the PP method is a method for detecting a deviation of the optical axis 101 in a radial direction 112 (a direction perpendicular to the tangential direction 107) towards pit sequences of the optical disc 102, by detecting a difference in the amount of light between the reflected beam 105 having entered the first area 106a and the reflected beam 105 having entered the second area 106b. 
With the PP method, the tracking error signal TE is calculated using the following equation using the signals shown in FIG. 4, in a manner similar the calculation of the focus error signal FE by the SSD method.TE=(R+i+R−o)+(R−i+R+o)−(L+i+L−o)−(L−i+L+o)  Equation 2
As understood from the above description, the signals used in the PP method are the same as those used in the SSD method, the signals being obtained by dividing each of the light-sensitive areas 109a+, 109a−, 109b+, and 109b− into three areas. Accordingly, when arithmetic circuits explained with reference to FIG. 7 are provided on the light-sensitive element substrate 104, the signals (L+i+L−o), (R+i+R−o), (L−i+L+o), and (R−i+R+o) can be used commonly to detect the focus error signal and the tracking error signal by performing different external operations, with use of the SSD method and the PP method, respectively.
On the other hand, to detect the tracking error signal by the PP method in the case shown in FIG. 8, an excessive number of operations are performed by the arithmetic circuits of the light-sensitive element substrate 104. This requires addition of arithmetic circuits and external output terminals, canceling out the merits of reducing the number of external output terminals.
As explained up to now, although it is possible for the SSD and PP methods to share signals in case a certain condition is met, it is difficult for the DPD method to share signals with other methods. That is to say, except for a case where no arithmetic circuit is provided on the light-sensitive element substrate 104, it is impossible for all the methods to share the signals output from the external output terminals, and it is impossible to prevent the arithmetic circuits from becoming complex or prevent the external output terminals from increasing in number.